1. Field of Invention
Embodiments of the invention relate generally to power management, and more particularly, to dynamic load adjustment per phase of a polyphase uninterruptible power supply.
2. Discussion of Related Art
An uninterruptible power supply (UPS) is used to provide backup power to an electrical device, or load, while the primary power source, or mains, is unavailable. A conventional UPS rectifies input power provided by an electric utility using a power factor correction circuit (PFC) to provide a DC voltage to a DC bus. The rectified DC voltage is typically used to charge a battery while mains power is available, as well as to provide power to the DC bus. In the absence of mains power, the battery provides power to the DC bus. From the DC bus, an inverter generates an AC output voltage to the load. Since the DC bus is always powered either by mains or the battery, the output power of the UPS is uninterrupted if the mains fails and the battery is sufficiently charged.
While uninterruptible power supplies are typically used to provide a continuous source of power at a local level (e.g., at a data center or in an office or home), such devices primarily depend on utility power due to the limited capacity of the batteries or other energy storage devices used in conjunction with the UPS. Historically, electric utility service has been demand-driven and inefficient, but increasingly technology is being employed to improve the availability, reliability and efficiency of the electric grid. Commonly, these improved electric grids are called “smart grids.”
A “smart grid,” as described by the Institute of Electrical and Electronics Engineers (IEEE) of New York, N.Y., includes “a next-generation electrical power system that is typified by the increased use of communications and information technology in the generation, delivery and consumption of electrical energy.” As of 2011, a smart grid is considered an overarching and evolving concept that is not limited to any particular technological features, and may also include standards, goals, objectives, and procedures that collectively support the development, implementation, and efficient delivery of reliable, affordable, and sustainable electricity services. Accordingly, no standard definition of smart grid presently exists; rather, the term refers broadly to various interrelated power generation, distribution, and consumption concepts. Some existing electrical power systems presently incorporate one or more elements that may be characterized as forming portions of a smart grid system rather than the whole.
Among other goals, one objective of a smart grid is to intelligently match power generation with power consumption. Historically, electric utilities have relied on total load averages, which are relatively stable and can be computed ahead of time, to anticipate power demands based on reasonable expectations of consumer requirements. The total load average is used to determine a baseload or minimum amount of power that a utility needs to make available without risking an interruption of service.
However, the total load connected to a power grid can vary significantly over time; for example, commercial power usage is typically higher during daytime than at night, residential power usage is highest during the early morning and evening hours, and loads also increase during periods where heating or cooling demands are greatest. Therefore, by using a total load average, the baseload (and any additional power generation kept in contingency) may be higher than necessary to support loads that fluctuate in real-time. Accordingly, highly reliable power delivery is achieved at the expense of wasted power that is generated in excess of demand. Furthermore, there may be times when the load increases substantially above the baseload capacity of the power utility with little or no warning, which introduces additional reliability and efficiency costs. Accordingly, one objective of a smart grid, generally, is to reactively and/or proactively adjust the baseload (e.g., increase or decrease supply to meet demand) and/or the total load (e.g., increase or decrease demand to meet supply) to optimally match the amount of power generated with the amount of power consumed (demanded) at any given moment. Today many power distributors attempt to match the power generated with the power consumed by stepping up or stepping down individual phases in the power distribution transformers (away from the power consumers); however, this only helps in outbalancing any unbalance in the line voltage and does not balance the grid load nor help reduce the unwanted losses in the distribution lines caused by the unbalance in the grid current. Therefore, it can be difficult to accomplish the power management objectives of the smart grid without some degree of cooperation between the electric utility and power consumers.